With respect to a method for forming a reduced diameter portion on an end portion of a metallic tubular member like a cylinder (hereinafter called as tubular member), in Patent document 1 for example, there is disclosed a method for forming a reduced diameter portion in any one of an offset, oblique or skewed relationships to a central axis of the tubular member by a spinning process. This is a sequential process for providing a desired shape by a plurality of cycles (a plurality of paths) of the spinning process. It is described that in the case where an oblique portion or a skewed portion (=nonparallel portion), the spinning process is achieved by providing a forming target axis for each path, holding a workpiece to mate the forming target axis with a revolution center (movable) axis of a roller, and the revolution center moving along the revolution axis with a revolution diameter of the roller being adjusted, whereby a desired oblique or skewed shape can be provided.
Patent document 1: Japanese Patent No. 3390725